The invention relates to semiconductor test equipment. More specifically, the invention relates to semiconductor probe card analysis, rework, and scrub mark analysis stations.
A variety of equipment and techniques have been developed to assist manufacturers of integrated circuits in testing those circuits while still in the form of dies on semiconductor wafers. In order to quickly and selectively electrically interconnect contact pads on each die to the electrical test equipment, arrays of slender wires or other contact media are provided. The contact media are arranged on conventional printed circuit boards so as to be positionable on the metalized contact pads associated with each semiconductor die. As is well known by those of ordinary skill in the art, those printed circuit board test cards have come to be known as xe2x80x9cprobe cardsxe2x80x9d or xe2x80x9cprobe array cardsxe2x80x9d.
As the component density of semiconductor circuits has increased, the number of contact pads associated with each die has increased. It is now not uncommon for a single die to have upwards of 600 pads electrically associated with each die. The metalized pads themselves may have as little as a ten xcexcm gap therebetween with an on-center spacing on the order of 50 xcexcm to 100 xcexcm. As a result, the slender probe wires of the probe array cards have become much more densely packed. It is highly desirable that the free ends or xe2x80x9ctipsxe2x80x9d of the probes be aligned in a common horizontal plane, as well as have the proper positioning with respect to one another within the plane so that when the probes are pressed down onto the metalized pads of an integrated circuit die, the probes touch down substantially simultaneously, and with equal force while being on target. As used herein, the terms xe2x80x9ctouchdownxe2x80x9d, xe2x80x9crestxe2x80x9d and xe2x80x9cfirst contactxe2x80x9d have the same meaning. In the process of making electrical contact with the pads, the probes are xe2x80x9cover traveledxe2x80x9d causing the probes to deflect from their rest position. This movement is termed xe2x80x9cscrubxe2x80x9d and must be taken into account in determining whether the rest position and the over travel position of the probes are within specification for the probe card.
The assignee of the present invention has developed equipment for testing the electrical characteristics, planarity and horizontal alignment, as well as scrub characteristics of various probe cards and sells such equipment under its Precision Point(trademark) line of probe card array testing and rework stations. A significant component of these stations is a planar working surface known as a xe2x80x9ccheckplatexe2x80x9d. A check plate simulates the semi-conductor die undergoing a test by a probe card while checking the above described characteristics of the probes. A suitable check plate for use with the assignee""s Precision Point(trademark) equipment is described in detail in U.S. Pat. No. 4,918,379 to Stewart et al. issued Apr. 17, 1990, the disclosure of which is incorporated herein by reference. It is sufficient for the purposes of this disclosure to reiterate that while the subject probe card is held in a fixed position the check plate is moved horizontally in steps when testing the horizontal relative positioning, and vertically in steps when testing the touchdown contact and over travel position of each probe tip. Previously, and as described in the above-identified patent, horizontal position information for each probe tip was determined by translating an isolated probe tip in steps across resistive discontinuities on the check plate. In recent years, this technique has been altered by placing a transparent, optical window in the surface contact plane of the check plate with a sufficiently large surface dimension so as to permit a probe tip to reside thereon. An electronic camera viewing the probe tip through the window digitizes the initial touch down image of the probe, and a displaced position of the probes due to xe2x80x9cscrubxe2x80x9d as the check plate is raised to xe2x80x9cover travelxe2x80x9d the probe. The initial touch down position is compared to the anticipated touch down position to assist an operator in realigning that particular probe.
Another prior art technique for determining relative probe tip positions in a horizontal (e.g. X-Y) plane is described in U.S. Pat. No. 5,657,394 to Schwartz et al., the disclosure of which is incorporated herein by reference. The system disclosed therein employs a precision movement stage for positioning a video camera into a known position for viewing probe points through an optical window. Analysis of the video image and the stage position information are used to determine the relative positions of the probe points. In systems of this type, a xe2x80x9creferencexe2x80x9d probe position is determined primarily through information from the video camera, combined with position information from the precision stage. If the pitch of the probes on the probe card is small enough, two or more probes can be simultaneously imaged with the video camera. The position of this adjacent probe is then referenced with respect to the xe2x80x9creferencexe2x80x9d probe from information from the video camera only. The camera is then moved to a third probe, adjacent to the second probe and this process is repeated until each probe on the entire probe card has been imaged.
Each of the above probe position determining methods suffers from its own, unique limitations. The method described in the ""374 patent to Stewart et al. relies heavily on the repeatability and accuracy of the stage which translates the check plate with respect to the probe pins. Although the position of each probe tip is determined uniquely with respect to a reference position of the stage, there are inherent limitations as to the resolution of the stage (i.e. the size of the smallest linear increment which a micropositioning device can move the stage under electronic control). Thus, this technique is not applicable if the desired tolerance of probe pin position is less than the resolution of the micropositioning stage.
The method disclosed in the ""394 to Schwartz et al. is theoretically capable of much greater accuracy because once the position of the xe2x80x9creferencexe2x80x9d probe has been determined, the position of every other probe tip in the probe tip array is determined relative to one another using the resolution of the video (typically CCD) camera. Modem CCD arrays can have picture elements (i.e. pixels) having on center distances on the order of 7.0 xcexcm or less. Thus, the resolution of this system is very high. Nevertheless, there are two principle limitations involved with this technique. The first limitation relates to a small error associated with each measurement made by the CCD array. These errors are cumulative for each subsequent probe measured in sequence. Thus, in an array comprising 600 or more probes, the positional measurement error of the 600th probe can be quite large. An obvious solution to this problem is to provide a CCD array which is capable of imaging all of the probe tips simultaneously. Unfortunately, the size of printed circuit probe card arrays (i.e. the number of probe tips per probe card) is increasing more rapidly than is the size of CCD arrays. As is well known to those of ordinary skill in the semiconductor manufacturing art, the difficulty in manufacturing larger semiconductor dies increases geometrically with the area of the die, whereas the ability to increase the pitch of printed circuit probe cards is not thusly constrained. The second limitation relates to the inherent accuracy of the optical system which forms the image of the probe pin tips. The ""394 patent does not disclose any technique for compensating for optical aberrations or alignment in accuracy in the optical system.
In an attempt to address the problems outlined above, the assignee of the present invention manufactures a probe card analysis and rework station under the designation PRVX(trademark) which uses a video technique to image printed circuit probe card array probe tips while referencing each measurement to the position of the stage rather than an adjacent probe tip. Thus, the Cartesian horizontal position determination of each probe is equally accurate. Nevertheless, modern printed circuit probe cards having probe densities exceeding 600 probes are approaching the limits of resolution of such a hybrid system. It is clear that at some point, both the size of printed circuit probe card arrays and the pitch density of probe tips will exceed the capabilities of such systems. In addition, as described above there are inherent mechanical limitations to the accuracy, repeatability, and resolution of the mechanical stages on which all of the prior art systems rely at some point during the mensuration process. For example, mechanical systems of this type are inherently subject to dimensional changes due to temperature fluctuations, mechanical wear, friction, and the like. To some extent, these variables can be compensated by numerical methods or position encoders. Nevertheless, a need exists for a printed circuit probe card analysis system having improved resolution, accuracy and repeatability over time.
It is therefore an object of the present invention to provide a method and apparatus for determining the relative positions of probe tips in a probe card array having high resolution, repeatability and accuracy.
It is a further object of the present invention to provide a method and apparatus for determining the relative position of probes in a probe card array which is not dependent on the accuracy of a mechanical stage.
It is yet another object of the present invention to achieve the above objects in a method and apparatus for determining the relative positions of probes in a probe card array which requires a minimum number of touchdowns of individual probe tips on a probe card checkplate.